This invention is related to a replenisher for palladium and palladium alloy electroplating baths. More specifically, this invention relates to a novel method for making palladium tetraammine sulfate as a replenisher for palladium and palladium alloy plating solutions.
Palladium complexes utilized in conventional palladium and palladium alloy plating chemistries contain by-products of their syntheses such as, for example, chloride and nitrite. These species can, under certain conditions, contaminate plating baths resulting in low efficiencies, fast degradation of organic components, and unstable bath performance. These specific are implicated in the corrosive attack of plating line equipment.
Precipitation of palladium salts has been a method used in the prior art to separate palladium from contaminating compounds. One method of precipitating palladium has been described by Rogin et al. (1990) Russian Journal of Inorganic Chemistry 35(5). In this particular method, palladium hydroxide is precipitated. The precipitated palladium hydroxide is reacted with oxalic acid to form an oxalate complex of palladium. The action of concentrated ammonium hydroxide on the palladium oxalate complex produces a complex of tetraammine palladium and palladium oxalate. Another method is described by Blokhin et al. (1989) Russian Journal of Inorganic Chemistry 34(6). In this particular method, a cationic palladium complex and palladium tetrachloride anion are reacted together and result in the formation of a so called xe2x80x9cpink salt.xe2x80x9d In both methods, chloride and oxalate ions are produced. These ions are electrochemically active species which adversely affect bath performance in the manner previously described.
Recently, it was discovered that palladium tetraammine sulfate complex that is 35 to 45 weight percent palladium could be a suitable replenisher for palladium ans palladium alloy electroplating baths when added with a sulfate complex of the alloying metal at high temperatures.
There remains a need to optimize bath performance by minimizing bath corrosiveness and prolonging the electroplating bath lifetime, as well as providing a replenishing complex that requires less palladium to produce.
This invention provides a palladium tetraammine sulfate complex which is from about 35 to about 45 weight percent palladium. For purposes of the present invention, the palladium complex is prepared by precipitating the complex from a supernatant solution and separating the complex from the solution. Because of the process by which the complex is prepared, the palladium sulfate complex contains a negligible amount of impurities (i.e. molecules or atoms that are present in the complex due to the presence of impurity or as by-products of the complex formation). A negligible amount of other moieties is a quantity of impurity that does not adversely affect bath performance. The complex is suitable for use as a replenisher of palladium in palladium electroplating baths. The palladium complex is added to the bath in an amount to maintain the concentration of palladium in the bath within from about 5 to about 10 weight percent of the recommended levels.
The complex of the invention is also suitable for use as a liquid or solid replenisher for palladium alloy electroplating baths when added in conjunction with a sulfate complex of the alloying metal.
In a further embodiment, a novel and more economical process for making the useful palladium complex is disclosed. An amount of palladium metal is provided and activated, preferably by washing with hydrochloric acid. The activated palladium is contacted with an excess of nitric acid to make a palladium nitrate solution. The solution is distilled at a temperature less than or equal to about 115xc2x0 C. An amount of palladium sulfate is added to the solution along with an amount of ammonium hydroxide. The solution is ultimately treated, for example by filtering, to isolate the product from the supernatant fluid.
The palladium tetraammine sulfate complex is preferably used as a replenisher for a palladium plating solution bath. The complex preferably contains from about 35 to about 45 weight percent of palladium, and the complex is added to the bath to maintain a palladium concentration in the bath of from about 5 to about 10 weight percent of recommended bath levels.
The electroplating process is preferably used to plate a metal or treated non-metal article. The complex is preferably rerpresented by the chemical formula [Pd(NH3)4]SO4.xH2O, where x is 0 in the non-hydrated form and x is a number from 1 to 6 in the hydrated form.
The foregoing is further exemplified from reading the specification and claims.